[RFI] A fun and educational RFI toy.

Larry Benko xxw0qe at comcast.net
Wed Feb 29 12:49:30 PST 2012

Be careful with LISNs and preamps.  Turning on and off the DUT (device 
under test) will create significant transients to the receiver.  I have 
never hurt a receiver or spectrum analyzer but have killed a couple of 
preamps.  The safe mechanism is to unplug the receiver from the lisn, 
turn on the DUT , and then reconnect the receiver.  I have seen a DUT 
that draws 10A produce spikes of 30V at the receiver when turning off 
the DUT.  Large interfering sources will be S9+20 at the receiver (FCC 
Part 18 limits for lighting devices is -59dBm) and no preamp will be 
needed.  However for quiet devices that you are trying to make even 
quieter you might need a preamp and most low noise preamps are not well 
protected against transients.

Larry, W0QE

On 2/29/2012 1:33 PM, KD7JYK DM09 wrote:
> I picked up a LISN, Line Impedance Stabilization Network box from a fellow
> many years ago.  This box allows me to apply power to a device under test
> and monitor any noise the device produces on the power line on a spectrum
> analyser.  They are used for electromagnetic compliance testing.
> Now for the fun part!
> These boxes are very simple in design and you can connect the output to your
> receiver.
> You can plug any device operating on household power to the box, connect it
> directly to your radio and all you'll hear in your receiver is the garbage
> your item produces over the power lines and possibly radiates into the
> neighbourhood.  Want to hear what a lightbulb produces, plug on in, fire up
> your receiver.  How about a power supply for your computer?  Plug it in,
> spin the dial.  Doorbell transformer with an odd smell?  Plug it in, fire up
> your receiver, listen to the hash and arcing...  How about the noise another
> receiver produces to the first?
> It's great for learning what everything you own, and only what that device,
> generates.
> Basic info:
> http://en.wikipedia.org/wiki/Line_Impedance_Stabilization_Network
> A search on Google brings up some designs for those that want to roll their
> own, there's a few on ebay, but they must be full of cocaine or gold bricks,
> the cheapest is about twenty times what I paid for a used one from a govt'
> contractor used to seeing big $ signs.
> Kurt
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